Thermocouples have long been used as a pressure sensing device for partial vacuums. The thermocouple is heated and the resulting temperature of the thermocouple is measured using the thermocouple EMF. As the gas pressure falls, the rate of cooling of the thermocouple by the gas decreases. As a result, either the temperature of the thermocouple for constant heating current rises or the current needed to keep the thermocouple at constant temperature decreases.
Thermocouple pressure gauges of the prior art are limited in their range and sensitivity by the noise generated by simultaneous heating during measurement. Prior art thermocouple gauges are insensitive at low pressures and in the range of pressures 10-100 Torr.